1. Field of the Invention
The present invention relates to a memory card apparatus using an EEPROM (electrically erasable and programmable read-only memory) as a semiconductor memory and, more particularly, to a memory card apparatus suitably used for an electronic still camera or the like which is designed to convert a photographed optical image of an object to be photographed into digital image data and to record the converted data in a semiconductor memory.
2. Description of the Related Art
As is generally known, an electronic still camera apparatus has been developed. This apparatus is designed to convert a photographed optical image of an object to be photographed into an electrical image signal, convert the image signal into digital image data, and record the data in a semiconductor memory. In an electronic still camera of this type, a memory card having a semiconductor memory incorporated therein is designed to be detachably mounted in a camera main body so that the memory card can be handled in the same manner as a film for a general camera.
The standardization of memory cards for electronic still camera apparatuses is now in progress. As a built-in semiconductor memory, a large-capacity memory is required in order to record digital image data corresponding to a plurality of frames. As such a memory, an SRAM (static random access memory), a mask ROM, or an EEPROM capable of electrically recording and erasing data, or the like has been considered. A memory card using an SRAM has already been on the market.
A memory card using an SRAM is advantageous in that it can conform to the data arrangement of any kind of format, and has a high data write/read speed. However, a backup battery must be housed in the memory card to retain written data. Consequently, the storage capacity is reduced according to the space for housing the battery. In addition, since the SRAM is high in cost, the use of it is economically disadvantageous.
In order to eliminate the drawbacks of the SRAM, a great deal of attention has currently been paid to an EEPROM as a semiconductor memory used for a memory card. The EEPROM attracts attention as a recording medium replacing a magnetic disk. The EEPROM has its own advantages in that for example, the EEPROM needs no backup battery for retaining data. It also allows a reduction in cost of a chip itself. Owing to such advantages, the development of an EEPROM used for a memory card is progressing rapidly.
FIG. 1 shows the merits and demerits of a memory card (SRAM card) using an SRAM versus a memory card (EEPROM card) using an EEPROM for comparison. First, with regard to "backup battery" and "cost" as comparison items 1 and 2, as described above, the SRAM card requires a backup battery and is high in cost, whereas the EEPROM card requires no backup battery and allows a reduction in cost.
Next, with regard to "write speed" and "read speed" as comparison items 3 and 4, consider a random access mode common to the SRAM and the EEPROM, in which a data write/read operation is performed with respect to a byte or bit arbitrarily designated by an address, and a page mode unique to the EEPROM, in which a data write/read operation is performed in units of pages by designating a page consisting of a plurality of consecutive bytes (several hundred bytes).
In the random access mode, both the write and read speeds of the SRAM are high, whereas those of the EEPROM are low. In the page mode, since a large number of data corresponding to one page are written/read in/from the EEPROM, its write and read speeds are higher than those in the random access mode.
"Erase mode" as comparison item 5 is unique to the EEPROM but is not preset in the SRAM. More specifically, in the EEPROM, data cannot be newly written in an area in which data has already been written without erasing the previously written data first. Therefore, when data is to be written, this erase mode is executed. The erase mode includes a chip erase operation in which all the stored contents of the EEPROM are erased at once, and a block erase operation in which stored contents are erased in units of blocks, each (several kbytes) consisting of a plurality of pages.
With regard to "write verify" as comparison item 6, this mode is also unique to the EEPROM but is not present in the SRAM. Generally, in the EEPROM, data cannot be completely written by one write operation. For this reason, every time a write operation is performed with respect to the EEPROM, the written contents must be read out from the EEPROM to check whether the data is correctly written. This operation is performed in the write verify mode.
More specifically, data to be written in the EEPROM is recorded in a buffer memory in advance. After the data transferred from the buffer memory is written in the EEPROM, the written contents of the EEPROM are read out to check whether the readout contents coincide with the contents of the buffer memory. If an incoincidence (error) is determined as a result of the write verify operation, the contents of the buffer memory are written in the EEPROM again.
As is apparent from the above comparison, the EEPROM has unique merits which the SRAM does not have, e.g., the ability to operate without a backup battery, a low cost, and the ability to perform a data write/read operation in units of pages. However, the EEPROM has some demerits as well. For example, the EEPROM has low data write and read speeds in the random access mode, and requires modes which the SRAM does not require, e.g., the erase mode and the write verify mode.
If, therefore, the EEPROM is to be used as a semiconductor memory for a memory card in place of the currently used SRAM, improvements in various aspects need to be made to details so as to allow the EEPROM card to be used in the same manner as the memory card incorporating the SRAM. For example, by eliminating some drawbacks, of the EEPROM card such as low data write and read speeds, and the necessity for the erase and write verify modes, an SRAM-card-like device having an EEPROM card may be used.